Titanium silicalite-1 is known from U.S. Pat. No. 4,410,501. It is an efficient catalyst for the oxidation of organic substrates with hydrogen peroxide such as, for example, the epoxidation of olefins (EP 1 100 119), the ammoximation of carbonyl compounds (EP 0 267 362) and the hydroxylation of aromatic compounds (U.S. Pat. No. 4,396,783). The catalytic activity of different titanium silicalite-1 samples depends on the size of the primary crystals as well as the titanium content (A.J.H.P. van der Pol, Appl. Cat. A92 (1992) 113). Thus a decrease in catalytic activity with increasing size of the primary crystals is observed.
In order to produce the highest possible catalytic activity in the liquid phase, it is therefore advantageous to use titanium silicalite-1 with small primary crystals. On the other hand the separation of small solid particles from a liquid when the diameters have values of a few micrometers or less is relatively costly on an industrial scale. Isolation and thus possible recovery of catalyst particles is desirable due to the high cost of manufacture and their inherent value.
It is known that titanium silicalite-1 can be agglomerated in order to improve the solid/liquid separation procedure.
A process for agglomerating titanium silicalite-1 and for preparing a titanium silicalite-1 granulate is described in EP-A 0 200 260, EP-A 0 265 018 and U.S. Pat. No. 5,500,199. Here an aqueous solution of oligomeric silicon dioxide and tetra-n-propylammonium hydroxide is first prepared by hydrolyzing a tetraalkylorthosilicate with an aqueous solution of tetra-n-propylammonium hydroxide. The titanium silicalite-1 crystals are suspended in this solution and the suspensions obtained are then spray-dried. Spherical agglomerates with a diameter of 5 to 1000 .mu.m should be produced using this process. They are composed of titanium silicalite-1 crystals and oligomeric silicon dioxide, wherein zeolite crystals should be "entrapped by means of Si--O--Si bonds". Silicon dioxide sources other than tetraalkylorthosilicates, such as colloidal silicon dioxide or sodium silicates lead to products of lower quality (EP-A 0 200 260). A molar ratio of oligomeric silicon dioxide to titanium silicalite-1 of 0.05 to 0.11 is cited for the composition of the catalyst in EP-A 0 200 260. A ratio by weight of oligomeric silicon dioxide to titanium silicalite-1 of 5:95 to 20:80 is cited in EP-A 0 265 018 and U.S. Pat. No. 5,500,199, corresponding to a SiO.sub.2 concentration of 5 to 20 wt. %.
Titanium silicalite-1 granulates prepared in accordance with known processes have serious disadvantages. Thus, titanium silicalite-1 granulates prepared in accordance with U.S. Pat. No. 5,500,199, Example 8 (see Example 1 in this application) exhibit much lower activity in oxidation reactions with hydrogen peroxide, such as for example the epoxidation of propene, than non-granulated titanium silicalite-1 powder (see Example 2 in this application). It can be seen from the SEM photographs of this material (see FIG. 1) that the agglomerated titanium silicalite-1 crystals are coated with a relatively compact catalytically inactive layer. This means that the reactants have restricted access to the catalyst crystals. The result is reduced catalytic activity. In addition a large proportion of the agglomerates is in the form of bowl-shaped or spherical fragments. Considerably increased mechanical abrasion takes place at any edges on the fragments.
The known process for preparing titanium silicalite-1 granulates also has a number of disadvantages:
Separation of the titanium silicalite-1 crystals formed during crystallization from the mother liquor is very costly due to their small average size of less than one micrometer. PA1 The mother liquor contains residues of unreacted tetrapropylammonium hydroxide that must be disposed of. This is associated with high costs. PA1 In order to produce the suspension of "oligomeric silicon dioxide" acting as binder, additional amounts of the expensive raw materials tetraethylorthosilicate and tetrapropylammonium hydroxide are required.